1. Field of the Invention
The present invention relates to mobile radio communication systems which have fixed radio-based stations and mobile stations between which control signals are transmitted. In particular, the present invention relates to a control signal transmission system in which registration access signals are sent from the mobile stations to the fixed stations and in which paging signals are sent from the fixed stations to the mobile stations.
2. Description of Related Art
Radio telephone service, which has been in use for some time, has traditionally comprised a central site capable of transmitting to a limited number of mobile or portable telephones in a large geographic area via high-powered transmitters. For clarity and convenience, the terms "mobile station" and "mobile stations" will henceforth be used in this document to mean mobile or portable radio telephone stations. Because mobile station transmissions originated in generally low power transmitters, in prior art systems mobile station transmissions were generally received by a network of satellite receivers remotely located from the central site. The satellite receivers were operable to receive and relay mobile station transmissions to the central site for processing. In prior art systems only a limited number of radio channels were available, thus limiting the number of radio telephone conversations in an entire city to the specific number of available channels.
Modern cellular radio telephone systems have a comparatively large number of radio channels available. These channels can be effectively increased by reuse of the radio channels in a metropolitan area, such as the metropolitan area of a large city like Chicago or New York, by dividing the radio coverage area into smaller coverage areas called cells and by using low power transmitters and coverage restricted receivers. Cellular systems of this general type are further described in U.S. Pat. Nos. 3,906,166 and 4,268,722. The limited coverage area within a cell enables the radio channels used in one cell to be reused in another cell which is geographically separated from it according to a predetermined plan, such as the exemplary four-cell pattern shown and described in U.S. Pat. No. 4,128,740. In this four-cell pattern, each cell is assigned a subset of the available radio channels and reuse of the radio channels is accomplished by repeating the pattern throughout a metropolitan area.
A cellular system typically utilizes a pair of radio frequencies for each radio channel in each cell. Each cell is assigned at least one paging/access channel and several voice channels. The paging/access channel is employed to control the operation of the mobile stations by means of data messages transmitted to and received from the mobile station. Control functions performable include identifying the particular cellular system to the mobile stations, instructing mobile stations to tune to a voice channel where a conversation may take place, receiving requests for service from mobile stations, and enabling mobile registration. The latter function refers to the process by which the mobile stations identify themselves to the system. The data message and radio channel specifications for U.S. cellular radio telephone systems are set forth in Electronic Industries Association (EIA) Standard IS-3D implemented in accordance with 47 C.F.R. 22 and Federal Communications Reports and Orders. Copies of EIA Standard IS-3D may be obtained from the Engineering Department of the Electronic Industries Association at 2001 Eye Street, N.W., Washington, D.C. 20006.
Since each cell of a cellular system may be relatively small in size, the likelihood of a mobile station travelling out of one cell and into another cell is high. The process of switching the established call from one cell to another is known as a handoff. A cellular system may determine the need for a handoff by periodically measuring the signal strength of each active mobile station. If the measured signal strength is below a predetermined level, the cellular system can determine the availability of other channels in neighboring cells and transmits an instruction to the mobile station commanding it to tune to an available channel in any one such acceptable cell. Also, as the number of cellular systems increase, handoffs between different cellular systems may take place in order to maintain a call as a mobile station passes from the coverage area of one cellular system to another.
It should be recognized that a purpose of mobile registration is to permit calls to a mobile station to be automatically delivered even though the mobile station may be moving from place to place through a network of cellular systems. It should also be recognized that mobile station registration according to EIA Standard IS-3D is effected by means of an interaction between the cellular system and the mobile stations operating in its service area. This interaction is called "autonomous registration" and it is controlled by the cellular system through certain information transmitted to the mobile stations. This information is in the form of an overhead message train (OMT), which is transmitted on paging/access channels throughout a cellular system service area, normally once each second. The OMT includes a system parameter overhead message including station and registration related messages, and optionally, several other messages of which the registration identification message and the registration increment message are relevant to the autonomous registration process.
Registration may be enabled or disabled individually for each class of mobile station, e.g., home or roam (explained below), by means of control bits in the system parameter overhead message. The system parameter overhead message also contains the identification number of the serving cellular system from which the mobile station determines whether it is a "home" or a "roam" mobile station. Each mobile station contains, in its internal memory, an entry indicating the identity of its home cellular system and entries indicating a predetermined number, e.g., four, of cellular systems (which may include the home cellular system) in which it has most recently registered successfully, along with a value for each cellular system used to determine when it is scheduled to re-register in that cellular system.
In the mobile telephone systems used in North America, the United Kingdom and in other markets, twenty-one frequencies are allocated for the control channels. A two-bit digital color code (DCC) is used to differentiate control channels using the same frequency. It is thus possible to have up to 84 cells, each cell having a control channel with a unique set of frequency and DCC combinations.
In densely populated areas, subscriber demand may require more than 84 cells to provide adequate mobile telephone service. In these systems, identical control channels may exist within the coverage area of a single exchange or within the coverage area of several neighboring exchanges.
A registration access signal is sent over the control channel by the mobile station and it is used to localize the subscriber in the mobile telephone exchange network. This signal enables a visited exchange to update the roaming subscriber's home exchange with the new location of the mobile station. However, this signal may be overheard by other control channels that have identical parameters, resulting in one or more cooperating exchanges registering the subscriber as a visitor. This occurrence is known as a multiple access (usually with double access occurring) and may result in incorrect location data updating within the home exchange for the roaming subscriber.
Such incorrect data within a system inefficiently uses system assets and can lead to such problems as failure of calls terminating at a mobile station. Thus, the unsatisfactorily addressed existence of multiple access is a shortcoming and deficiency of heretofore designed and implemented mobile communications systems.
Although no prior art solutions to the aforementioned multiple access problem are known, a number of patents contain teachings that bear some relation to the matters discussed herein. These patents are U.S. Pat. Nos. 4,833,702, 4,178,476, 4,352,955, and 4,843,622. Each of these patents is discussed briefly immediately below.
U.S. Pat. No. 4,833,702 to Shitara et al. discloses a mobile telephone system having a central controller and a cordless telephone located in a local service area. The central controller transmits a periodic pre-registration polling signal to all cordless telephones which can receive it and registers telephones from which a response signal is returned. After a cordless telephone is registered, a post-registration signal is transmitted by the telephone at periodic intervals. The central controller cancels the telephone registration if the post-registration signal is not detected for a prescribed time period.
U.S. Pat. No. 4,178,476 to Frost relates to an automatic roaming paging system of the type wherein multiple subscribers are each provided with a subscriber receiver which responds to a uniquely coded radio transmission by providing a perceptible indication that the subscriber is being paged, and in which each subscriber is assigned to a home station which broadcasts each radio transmission through a prescribed geographical area served by that station. Specifically, Frost teaches a method including the steps of responding to a predetermined command originating from at least one of the subscribers by automatically transferring page data subsequently received at a home station of the one subscriber to a further station which serves a further geographic area for the one subscriber and for which page data is received at the further station; transmitting from the further station the uniquely coded radio transmission to which the receiver of the one subscriber responds; and automatically suspending transmission of the uniquely coded radio transmissions intended for a subscriber by the home station of the subscriber in response to receiving a suspend command from that subscriber.
U.S. Pat. No. 4,352,955 to Kai et al. relates to a control signal transmission system for use in a mobile radio communication system. According to the teachings of Kai et al., a control signal transmission system for mobile radio communications uses a plurality of control channels extending between a plurality of mobile units and at least one control center. The control center transmits systems information signals and paging signals during individually associated time frames. The mobile units transmit response signals corresponding to the paging signals by transmitting to the control signals through the control channels. At least one frame of the system provides for a transmission of responsive information signals, following each frame including the paging signals. Each of these response signals includes an identification number of the replying mobile unit.
U.S. Pat. No. 4,843,622 to Yotsutani et al. relates to a communication control system including a searching arrangement. Yotsutani et al.'s system uses a page sent on a control channel to locate a mobile subscriber. This system controls several cells. This system sends a page to a plurality of cells in a given area when it has a call to place. Upon receiving a response from a mobile in a given cell, addressing is begun to the mobile in that cell.
Review of each of the foregoing patents reveals no disclosure or suggestion of a solution such as that taught herein to the multiple access problem.